While you might think about your website or mobile app as a distinct, real-world piece of property, like a storefront, Analytics understands a property only as a resource associated with your tracking code. When you track a resource using Analytics, you include a property ID in the tracking code that you put on your web pages or in your app source code. Performance data, like number of users or screen views, for resources tagged with the same ID is collected into the corresponding property.
For example, if you associate one property ID with two websites, data for both websites appears in the same property in your Analytics account. You can then use views and filters to organize the data. You can segregate the data even further at the report level with tools like Segments.
Consider your long-term reporting goals and how they might affect your account structure. Create an implementation plan before you set up new properties. In some cases, you may want to have multiple resources all send data to the same property. In other cases, you may want each resource to send data to a separate property. In either case, you can use views and filters to create specific perspectives of your data.